The invention relates generally to autoradiography and more specifically to high resolution alpha-track autoradiography.
The term autoradiography is quite general and its use has been applied to numerous fields of research. Autoradiography by itself encompasses the automatic (auto-) or spontaneous production of an image (-graphy) by radiation (-radio-). The prefix "auto" implies that the necessary radiation comes from within the object being imaged (as opposed to "radiography" which is the production of an image by an external source of radiation such as x-rays). This can be induced, as in neutron-induced autoradiography with such reactions as (n,.alpha.), (n,p) or (n,f), or spontaneous, which is most commonly encountered by labelling the object or tissue with radioactively decaying isotopes such as the beta emitters, .sup.3 H, .sup.14 C, .sup.35 S, and .sup.32 P. Different labels on different drug compounds that would be incorporated into the object, such as an animal or a plant, produce different images, each which tells a separate story about the drug and its distribution within the sample. In many cases, investigators clarify the description by denoting the term as H-autoradiography or neutron-induced alpha autoradiography, for example.
Levels of resolution at which the test sample is to be imaged plays an important role in deciding which technique and/or radioisotope to use. Through the use of large microtomes, entire body sections can be cut to produce macroscopic or whole-body autoradiographs. At low magnification, microscopic or light microscopic autoradiography is performed and finally at extremely high resolution, electron microscopic autoradiography is used. All three levels of producing autoradiograms requires very different procedures and many times entirely different radioisotopes.
Unlike radiology and clinical imaging, autoradiography is a research technology to study detailed distributions of radiolabels for toxicological and pharmacological testing of drugs, new compounds, or environmental effluents that may enter the body. Aside from biological applications, it is also used to investigate doping of semiconducting materials, imperfections in crystals and even to investigate authenticity of oil paintings and the styles of the painters who painted them. In order to do so, an interface of the specimen is placed in contact with some medium that is capable of detecting the radiations coming from the sample. This medium which is used to produce the image or distribution of the radiation emitting compound has for the majority of the history of autoradiography been a photo-sensitive emulsion not unlike that which is used in standard photography and x-ray imaging. In this process, the sample is basically placed in contact with the photo-emulsion and protected from light. The radiation coming from various parts of the sample then sensitize the emulsion at those points similar to light exposing a photographic film. Over a certain period of time, the exposure is sufficient to have sensitized enough of the film and it is then developed, thus revealing the regions corresponding to high activity of the radioisotope. This is the basis for all autoradiography that has become known as conventional, emulsion, or isotopic autoradiography.
There is a need for choosing a detector medium that is sensitive enough to detect alpha particles but at the same time insensitive to protons. An appropriate medium that satisfies this condition is bisphenol-A polycarbonate (C.sub.16 H.sub.14 O.sub.3) available commercially as Lexan (General Electric Co., Pittsfield, Mass.). Its many properties, including for example flexibility and transparency, are very favorable for this application. Other Polymers such as Polyethylene terephthalate, cellulose acetate, and cellulose acetato butyrate, are also acceptable, but by far the most extensively studied in track detection has been Lexan, which is the detector used in the embodiment to be described.